This invention is related to processes for the electrochemical reduction or oxidation of organic halides.
Several types of electrochemical redox reactions are known including, for example, dimerizations, internal cyclizations, polymerizations and oxidative and reductive substitutions. Many of these types of reactions are useful in the preparation of pharmaceuticals, agricultural intermediates and other monomers useful in the production of specialty chemicals.
An example of an electrochemical redox reaction that has been well studied is the dimerization of organic halides. One example of such a reaction is the coupling of an allyl halide to form hexadiene. The problems concerning electrochemical reductive dimerizations include selective reduction of the halide functional group with respect to other functional groups, the solvent and the desired dimer.
Various approaches to the solution of this problem exist. U.S. Pat. No. 4,253,921 discloses a method for the electrochemical preparation of an .alpha., .omega.-polymethylene diol from the coupling of polymethylene halohydrin wherein the halogen of the halohydrin is iodine or bromine. Another process for the preparation of 1,4-butanediol by the electrochemical coupling of a halohydrin in a divided electrochemical cell having a copper cathode is disclosed in U.S. Pat. No. 4,324,625. Again, the halogen of the halohydrin is iodine or bromine.
Additional electrochemical reactions of organic halides including catalytic electrochemical reduction or nucleophilic substitution are discussed by Rollin et al. in J. Chem. Research (S), 322-323 (1981). The use of an unstable nickel species in a low valence state to catalyze the reactions is disclosed. The electrochemical coupling of organic halides in the presence of a Ni(II)/Ni(I)/Ni(0)-PPh.sub.3 catalyst system is discussed by Schiavon et al. in J. Chem. Soc. Dalton Trans., 5, 1074-1081 (1981). The electrochemical reductive coupling of benzylic and allylic halides using catalytic amounts of Cr(II) which is regenerated during the process is disclosed by Wellmann et al. in Synthesis, 901-902 (December, 1978). Each of these catalytic electrochemical reactions of organic halides show some utility when the halogen involved is iodine, bromine or chlorine.
Thus, electrochemical reactions involving organic halides are generally useful only (1) when the organic halides are limited to compounds containing iodine or bromine, or (2) when expensive and/or toxic catalytic systems are used.
U.S. Pat. No. 3,876,514 teaches that, in the electrolysis of olefinic halides to produce dienes, allyl chloride may be converted to allyl bromide so that the reaction may take place under the milder conditions needed in the bromide reaction Wawzonek et al., J. Electrochem. Soc., 111, 74 (1964) teach that organic chlorides are reduced at more negative potentials than corresponding iodides or bromides. They also teach that in some cases an organic chloride will undergo displacement with iodide or bromide ions to yield the organic bromide or iodide which will be reduced at the less negative potential.
However, in these situations, it is generally taught that at least a stoichiometric amount of the iodide or bromide ions must be present. Further, the replacement reactions wherein the chloride is exchanged with iodide or bromide are not completely efficient. Even when the partial exchange has been affected, the use of the milder conditions appropriate for the iodide or bromide reactant does not result in conversions and yields that would be expected if the reactant were completely in the form of an iodide or bromide.
Thus, what is needed is an efficient method of converting organic chlorides to organic bromides or iodides. Such a method used in conjunction with known electrochemical reactions would permit both the use of relatively inexpensive organic chlorides as starting materials and the milder reaction conditions associated with the reactions of organic iodides and bromides.